1. Field of the Invention
The present invention relates generally to a radar apparatus and, more particularly, to a tuning method used in a radar apparatus which transmits and receives signals having multiple pulselengths assigned individually to selected range scales through a single antenna.
2. Description of the Related Art
A radar apparatus conventionally used on a ship, for instance, transmits a specific pulse signal and receives an echo signal reflected by surrounding targets through an antenna. The radar apparatus determines angular direction, or bearing (.theta.), of a particular target from the direction of the antenna as well as a range (r) to the target based on the time elapsed from transmission of the pulse signal to reception of the echo signal.
The pulse signal transmitted from the antenna has a pulselength which is determined in accordance with a detection range and range discrimination that are required. Typically, the pulselength is determined for each range scale selected for on-screen presentation. For example, a short pulselength is selected to achieve higher range discrimination on a short range scale as echoes received from nearby targets are generally strong, whereas a long pulselength is selected to receive a larger amount of information and thereby achieve improved target detectability on a long range scale as echoes from far targets are weakened.
If the transmission pulselength is lengthened to improve long-range target detectability, however, the range discrimination deteriorates. Thus, there arises a drawback that nearby targets can not be discriminated from one another when these targets are observed on a long range scale. The conventional radar apparatus exhibits this drawback most conspicuously particularly when viewing nearby targets on a long range scale. For this reason, it has been necessary for a radar observer to switch the radar apparatus to an optimum range scale depending on the distance between own ship and targets of interest.
Although some radar apparatuses offer a “zoom” function which is conventional, the zoom function simply displays an enlarged picture obtained by “zooming in” part of the echo signal received on the range scale in use and, thus, the enlarged part of the echo signal is not displayed with increased range discrimination. Therefore, the zoom function does not provide any solution to the aforementioned drawback of the conventional radar apparatus.
Japanese Patent Application Publication No. 1992-98179 and Japanese Patent No. 3507717 each disclose a technique which enables a radar apparatus to transmit signals having different pulselengths assigned individually to two or more range scales according to a selected transmit pulse sequence and simultaneously display radar pictures on two or more range scales. This technique, if implemented in a radar apparatus, makes it possible to simultaneously present echo signals obtained with multiple pulselengths on different range scales with high target detectability.
The aforementioned technique of Japanese Patent Application Publication No. 1992-98179 and Japanese Patent No. 3507717 however has a problem that a receiving circuit employing a conventional tuning method can not perform filtering operation at a sufficiently high signal-to-noise ratio (SNR). This is because the radar apparatus employing the technique successively switches the pulselength at high speed according to the transmit pulse sequence.
This problem of the aforementioned multiple pulselength technique is now examined in detail.
A commonly known conventional radar apparatus successively transmits pulses having a single pulselength which is determined in accordance with a detection range and range discrimination that are required. Thus, the pulselength of a pulse signal to which a receiving circuit of the radar apparatus should be tuned remains constant over time, so that the receiving circuit can normally execute tuning operation at a desired timing without the need to verify the pulselength of the signal. In performing the tuning operation, the receiving circuit sets a tuning voltage at the earliest timing according to task priority in order that a tuning voltage value given by a signal processor will be achieved as soon as possible.
Since the receiving circuit once tuned does not become greatly mistuned in actuality, the receiving circuit does not perform the tuning operation at every transmit pulse to alleviate work load of a mathematical processor of the radar apparatus. Rather, the receiving circuit performs the tuning operation at specified time intervals regardless of transmit signal output timing.
If the tuning operation of this kind performed in the conventional radar apparatus is used in combination with the aforementioned technique of Japanese Patent Application Publication No. 1992-98179 and Japanese Patent No. 3507717, it would be impossible to control the tuning voltage in such a way that the tuning voltage varies in accordance with the frequently switched pulselength of the transmit signal, and this would result in the aforementioned problem that an echo signal having a sufficient SNR is not obtainable.
It is to be pointed out that even the conventional radar apparatus switches the receiving circuit from one transmit signal pulselength to another when the range scale is altered. Operation carried out by the conventional radar apparatus when the pulselength is changed is as follows. First, the radar apparatus gives operational parameters corresponding to the newly selected transmit signal pulselength to individual processors and the individual processors perform signal processing suitable for the new pulselength according to the given operational parameters.
Then, a tuning processor generates a tuning voltage value corresponding to the newly selected transmit signal pulselength using a main bang signal obtained by transmission and reception of the pulse signal having the new pulselength and performs operation for setting and generating a new tuning voltage.
The aforementioned tuning operation however requires an extremely long period of time compared to a transmission interval of the pulse signal. Thus, if the above-described conventional tuning operation is used in combination with the technique of Japanese Patent Application Publication No. 1992-98179 and Japanese Patent No. 3507717, the aforementioned problem that an echo signal having a sufficient SNR is not obtainable would result.